Fig. 4

Quantum mechanical parameters (semi-empirical PM3 method) of different symmetrical MACs (bis (ortho-substituted 2,6-dibezylidenecyclohexanones), including C66 (X = CF₃). A. General structure of a symmetrical MAC [(2E,6E)-2,6-bis(2-X-benzylidene)cyclohexanone], where X is the substituent atom/group (e.g., Br, CF₃, NO₂); q_1,2,3 represents charge density on the respective atom; and θ_1,2 is the torsional angle between the respective bonds. B. Graph showing the charge density of the beta-carbon of the enone, q₃, depending on the substituent atom/group. The presence of a strong electron-withdrawing substituent leads to a decrease in the charge density, which in principle makes the MAC a better Michael acceptor. C. Graph showing the energy of the lowest unoccupied molecular orbital, E_LUMO, as a function of the substituent atom/group. E_LUMO is related to the electron-accepting capability of the MAC